Field of the Invention
This invention relates to a hollow-core waveguide with an optimized contour that makes it possible in particular to reduce transmission losses. It relates more particularly to a hollow-core photonic crystal fiber with an optimized contour for a transmission with low linear losses, a modal content that approaches and equals the monomodal speed, and finally a very poor recovery of the mode field guided with the material that constitutes the hollow-core contour.
Description of the Related Art
The photonic crystal fibers (FCP or PCF in English for “photonic crystal fiber”) are waveguides formed by a periodic two-dimensional network of inclusions that extend over the entire length of the fibers. Owing to their structures, these fibers ensure that the electromagnetic waves are confined in the fiber core.
These photonic crystal fibers offer a large variety of possibilities for guiding by adjusting their opto-geometric parameters, such as, for example, the diameter of inclusions, the distribution of inclusions, the periodicity (step between two inclusions), the number of layers, and the index of materials used. Multiple techniques can be used for producing these photonic crystal fibers.
According to a first so-called stacking and drawing operating mode (in English, “stack and draw”), capillaries in the form of hollow tubes of 1 to 2 mm in diameter are assembled in a bunch. This bunch is then inserted into a tube for ensuring that the capillaries are held. The tube and the bunch of capillaries are then drawn.
In the case of a hollow-core photonic crystal fiber, the center does not comprise capillaries so as to form the hollow core.
According to another operating mode, a cylinder can be machined in such a way as to generate the inclusions and is then drawn.
The photonic crystal fibers comprise solid-core photonic crystal fibers and hollow-core fibers (HC-PCF fibers).
This invention relates more particularly to hollow-core photonic crystal fibers.
By way of example, the document U.S. Pat. No. 7,315,678 describes a waveguide in the form of a hollow-core photonic crystal fiber comprising a sheath with a triangular structure, with the absence of a capillary in the central part forming the hollow core. Before the drawing phase, the hollow core is delimited by a series of arcs that correspond to the walls of the capillaries that delimit the hollow core. According to a particular feature of this invention, before the drawing phase, the hollow core, in a cross-section, has a non-circular shape with a first dimension in a first direction that is larger than a second dimension in a second direction. During the drawing phase, the arcs that form the contour of the hollow core have a tendency to become flattened, although the hollow core has an approximately circular cross-section after the drawing phase. According to this document, the fiber that is described makes it possible to obtain low losses, on the order of 1 dB/m for a range of wavelengths extending from 1 mm to 1.5 μm and whose guiding is optimized for the spectral range of “terahertz.”
Based on the nature of the guiding, the photonic crystal fibers comprise photonic band gap-guided fibers (BIP fiber or PBG fiber in English for “photonic band gap fiber”) as well as fibers with inhibited coupling guiding (in English, inhibited coupling guiding HC-PCF), also called large-pitch fibers (in English, large-pitch HC-PCF), or Kagome-type fibers.
The hollow-core fibers with photonic band gap guiding according to the prior art have the following drawbacks:
Even if for certain wavelengths on the order of 1.5 μm, it is possible to obtain transmission losses on the order of 1 dB/km, this value increases very quickly for shorter wavelengths. Thus, for the wavelengths of the visible range, the transmission losses can reach 1,000 dB/km.
According to another problem, these fibers with photonic band gap guiding have a bandwidth on the order of 70 THz that may prove too narrow for certain applications such as non-linear optics, the guiding of very short laser pulses.
According to another problem, the hollow-core fibers with photonic band gap guiding of the prior art have a dispersion that is too high and structured for certain applications such as high-resolution spectroscopy or the guiding of very short laser pulses.
Finally, according to another problem, the hollow-core fibers with photonic band gap guiding of the prior art have a strong recovery of power between the mode guided in the core and the contour that produces a low laser damage threshold.
The fibers with inhibited coupling guiding have the advantage of obtaining an ultra-wide bandwidth in contrast to the hollow-core fibers with photonic band gap guiding. In return, these fibers have higher transmission losses than those of the hollow-core fibers with photonic band gap guiding, greater than 0.5 dB/m.
The fibers with inhibited coupling guiding may have different structures relative to the sheath.
According to a first so-called triangular structure, the network of inclusions comes in the form of capillaries with circular cross-sections, distributed over a hexagonal surface, with the centers of the capillaries being arranged in such a way as to form equilateral triangles.
According to a second structure called a Kagome-type structure, the network of inclusions is delimited by walls that are parallel to the direction of propagation of the fiber and oriented in three directions at 60° in such a way as to isolate channels with hexagonal cross-sections and others with triangular cross-sections. According to this arrangement, the three walls are not concurrent at the same point.
By way of example, the document WO2009/044100 describes a fiber with inhibited coupling guiding crystals with a Kogame-type structure.
To obtain this type of guiding, after the drawing phase, the walls of inclusions should be the finest possible and the longest possible between two consecutive nodes, which corresponds to large-pitch structures.
The publication “Low Loss Broadband Transmission in Hypocycloid-Core Kagome Hollow-Core Photonic Crystal Fiber” Mar. 1, 2011/Vol. 36, No. 5/OPTICS LETTERS” proposes a fiber with inhibited coupling guiding with a Kagome-type structure and a hollow core. More particularly, this document describes two fibers: a first fiber with an approximately circular core and a second fiber with a core that is delimited by a contour that comprises a series of arcs. According to this document, these arcs form a hypocycloidal contour.
In contrast to the first fiber that generates a transmission loss on the order of 1.4 dB/m, the second fiber generates a transmission loss on the order of 0.4 dB/m.
According to this document, it is possible to obtain a fiber that generates a transmission loss on the order of 180 dB/Km with a bandwidth on the order of 200 THz.